This invention relates generally to firing circuits for thyristor power conversion systems and more particularly to an improved means for controlling the operation of a thyristor converter AC motor drive system including a polyphase synchronous motor supplied from a polyphase alternating current source.
Many circuits and systems are known for controlling the conductivity of controlled rectifiers utilized in various types of converters for supplying electrical power to a load, such as an AC motor, from a polyphase alternating current (AC) source. The type of rectifier used generally controls the type of control utilized, but by far the most common controlled rectifier used today is the thyristor of the silicon controlled rectifier type. A thyristor becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to its gate electrode and thereafter reamins conductive until the anode current falls below the value required to hold the thyristor in the conductive state.
Whereas motor control systems employing thyristors are known which have been implemented using analog control techniques, a typical example being U.S. Pat. No. 4,230,979, entitled, "Control Current Inverter and Motor Control System", Paul M. Espelage, et al., Oct. 28, 1980; attention has been and is presently directed to digital type of control techniques, examples of which are disclosed in: U.S. Pat. No. 3,601,674, entitled, "Control System For Firing SCR's In Power Conversion Apparatus", John A. Joslyn, et al., Aug. 24, 1971; U.S. Pat. No. 4,263,557, entitled, "Power Converter Control", Willard B. Jarvinen, Apr. 21, 1981; and U.S. Pat. No. 4,276,505, entitled, "Microcomputer Base Control Apparatus For A Load Commutated Inverter Synchronous Machine Drive System", Bimal K. Bose, June 30, 1981. The teachings of these patents are also intended to be incorporated herein by reference for enabling one skilled in the art to understand the present invention without the disclosure of extraneous and irrelevant material.
In a load commutated inverter (LCI) synchronous motor drive typically including a source side AC to DC converter and a load side DC to AC converter coupled together by means of a DC link circuit for supplying AC power to a motor load from a polyphase AC source, the torque developed by the motor is a function of stator current which is supplied by the load side inverter and the DC link circuit as well as the field current, the latter being supplied from a field excitation source. For a given field current, the torque increases with stator current until a peak is obtained. Thereafter further stator current increases result in a decrease in torque due to the excessive demagnetization of the field caused by the increasingly leading power factor necessary to obtain thyristor commutation. Additionally, the torque peak with stator current is also a function of field current; however, a direct measure of field current is generally not available.
Accordingly, during normal operation of an LCI motor control system, it is desirable to limit the stator current to a value corresponding to the peak torque region for the field current applied and failure to provide such as limit can result not only in a reduction in operational efficiency, but also in undesirable control ambiguity.